The petroleum and refinery industries use a large number of gate valves for controlling flow of liquid and gas products. An earlier retractable seal gate valve is illustrated in U.S. Pat. No. 2,977,086, Heinen; this valve is closed by a sequential motion including a downward vertical movement of a wedge-slip assembly through a lateral passageway to a position in a fluid-passageway between inlet and outlet ports of the valve, followed by a horizontal movement of the sealing slips into engagement with the respective adjacent valve seats. The open position is achieved by a reverse sequential movement, wherein the slips are first moved at a right angle away from the respective port seats and thereafter the wedge-slip assembly is moved in an upward vertical direction through the lateral passageway away from the fluid passageway of the valve.
Gate valves of this general type sometimes employ diametrically-opposed guide rails which extend longitudinally of the inside face of the lateral passageway with the wedge-slip assembly being moveably held by grooves at opposite side edges of the wedge to the guide rails of the valve body. Wedge-slip valves, perhaps because of their sequential opening and closing motions and complexity of parts, require a stringent engineering design to assure reliability and trouble free operation.